Pneumatic piston-cylinder assemblies are generally known and have utility in a number of different applications. For example, welding systems, such as spot welding guns, employ pneumatic piston-cylinder assemblies to move and hold electrodes at a desired location during a welding operation. As is well known by those having ordinary skill in the art, spot welding is a form of welding wherein a work piece (e.g., two or more pieces of metal) are joined by passing a current between electrodes positioned on opposite sides of the metal pieces. The actual weld is made by the combination of: a) heat generated as the current passes through the metal; b) the pressure applied to the pieces at the weld location; and c) the time the heat and pressure are applied at the weld location.
Conventional control systems for spot welding guns, for example, utilize a single pressure source providing a common pressure for effecting motion of the piston-cylinder assembly. Such systems are referred to herein as single pressure systems. The pneumatic pressure from the pressure source, via control valves, is provided to different pressure chambers of the piston-cylinder assembly to effect one of several motions, e.g., a pre-stroke motion or a weld stroke motion.
As used herein, pre-stroke (or retract stroke) refers to the motion of the electrodes from an initial starting position to an intermediate position (or from intermediate position to starting position), wherein the tips of the welding apparatus may be near but not contacting the work piece. A weld stroke refers to the motion of the tips from the intermediate position to contact with the work piece, and a return motion refers to the motion from contact with the work piece to the intermediate position.
Typically, a first control valve controls pneumatic pressure to a first pressure chamber of the piston-cylinder assembly so as to effect the pre-stroke motion, while a second control valve controls pneumatic pressure to a second pressure chamber of the piston-cylinder assembly so as to effect the weld motion. Alternatively, individual piston-cylinder assemblies (e.g., separate and distinct assemblies) may be employed (e.g., one for the pre-stroke motion and one for the weld motion), wherein each valve controls pneumatic pressure to a respective piston-cylinder assembly. In both scenarios, however, the systems are single pressure systems.
In manufacturing environments, it is desirable to increase the speed at which processes occur so as to increase productivity. For example, during the assembly of an automobile or a component of an automobile, there may be a number of spot welds applied to the automobile/component. If the cycle time for each spot weld can be reduced, then more automobiles/components can be processed, thereby increasing productivity. Thus, it is desirable to move the electrode tips of the spot welding gun as fast as possible so as to minimize cycle times and increase productivity. However, bringing the electrode tips in contact with the work piece at a relatively high rate of speed can cause the tips to bounce off the work piece, which can shorten the life of the tips and/or reduce the quality of the weld.
To minimize the bounce of the electrode tips as they contact the work piece, it is preferable that the tips approach the work piece at a relatively slow speed. However, this tends to slow the overall cycle time of the welding process, which is undesirable. Further, to enhance the strength and/or quality of the weld, it is desirable to apply a significant force to the work piece at the weld location. The force can be created, for example, by applying relatively high pneumatic pressure to the piston-cylinder assembly. However, this high pressure can take considerable time to build up and/or remove from the piston-cylinder assembly, thereby slowing the cycle time of the welding operation, which also is undesirable.
As is evident, a dilemma exists between increasing productivity and enhancing quality of the spot weld. If one chooses to increase the quality of the weld and/or reduce electrode bounce, productivity suffers. Similarly, if one chooses to increase speed to enhance productivity, electrode bounce is increased and weld quality is reduced.